DNA damaging agents have long been understood to drive mutagenesis while also being useful as cytotoxic agents to chemotherapeutically treat cancers. The appropriate response to such insults—mitigating cellular toxicity and inducing repair or initiating cell death—is critical, particularly in multicellular organisms. In response to genomic damage, DNA damage response (DDR) proteins, including ATM, ATR, CHK1/2 and p53-regulated pathways, instigate many of the responses, particularly DNA repair. However, some of the pleiotropic responses to damage appear to involve processes other than the central DDR; for example activation of antioxidant systems, switches in metabolic flow, drug efflux, and phase I/II drug detoxification pathways also seem to take part in this intricate response network in a thus far not fully understood pattern (Ravi et al., 2009, PLoS Genet 5, e1000527; Altieri et al., 2008, Antioxid Redox Signal 10, 891-937; Workman et al., 2006, Science 312, 1054-9).
Even though alkylating agents are assigned as one of the most powerful carcinogenic DNA damaging agents, compounds such as cyclophosphamide, carmustine, melphalan, temozolomide, and alkylation-like platinum compounds such as cisplatin are still at the forefront of many chemotherapy regimens used to treat breast (Hernandez-Aya, Gonzalez-Angulo, 2013, Surg Clin North Am 93, 473-91; Jones et al., 1993, Breast Cancer Res Treat 26 Suppl, S11-7), gliomas (Omuro, DeAngelis, 2013, JAMA 310, 1842-50; Friedman et al., 2000, Clin Cancer Res 6, 2585-97) and lung cancer (Chan et al., 2013, J Thorac Dis. 5 Suppl 5, S565-78) as well as some autoimmune diseases (Kallenberg, 2013, Ann Rheum Dis 72 Suppl 2, ii62-5). Chemically, alkylation is the transfer of an alkyl group from one molecule to another, which biologically could be DNA, RNA, or protein. The primary mechanism of action of alkylating chemotherapies is thought to be through induction of nucleotide modification, which leads to base adducts, DNA crosslinks and strand breaks as well as reactive oxygen species (ROS) and the indirect damages this will cause (Lin et al., 2012, Free Radic Biol Med 52, 377-91; Drablos et al., 2004, DNA Repair (Amst) 3, 1389-407; Malet-Martino et al., 1999, Curr Pharm Des 5, 561-86). This implies impairment of the replication machinery, repair system activation, cell cycle arrest, altered transcription and/or activation of death mechanisms.
Besides cytotoxicity to cancer cells, off-target damage to healthy tissues is a frequently observed component leading to alkylating therapy failure and poor prognosis. Alkylating agents are known to cause severe damage to proliferating cells, mainly those of the immune system leading to immune suppression. However, such toxicity is not exclusive of proliferating cells since low/non-proliferating counterparts, including renal, hepatic, and cardiac tissues, are also severely affected by some alkylating agents (Abdelrahman et al., 2010, J Appl Toxicol 30, 15-21; Chen et al., 2008, Br J Pharmacol 153, 1364-72; Chen et al., 2007, Can J Clin Pharmacol 14, e246-50). For example, cyclophosphamide may cause severe renal failure and severe cardiotoxicity even in the first cycles of chemotherapy (Chen et al., 2007, Can J Clin Pharmacol 14, e246-50; Kusumoto et al., 2013, Intern Med 52, 2311-5), cisplatin (CDDP) may induce nephrotoxicity (Bayomi et al., 2013, Eur Cytokine Netw 24(4):139-47) and ototoxicity (Bass et al., 2014, Pediatr Blood Cancer 61, 601-5). Consequently, the harmful side effects of some alkylating agents can delay a chemotherapy protocol completion, which could favor the emergence of infections and resistant cancer phenotypes. Much of these undesirable effects are believed to be associated with production of ROS and oxidative damage to cellular structures but the mechanism is still not fully depicted (Chen et al., 2007, Can J Clin Pharmacol 14, e246-50).
While some preclinical studies have addressed the effect of antioxidants in minimizing off-target damages from alkylation, others have sought to identify targets that would potentiate chemotherapy toxicity or circumvent tumor cells resistance. However, there is still a key deficiency in understanding how normal cells survive alkylation and how the efficacy of alkylation in treating cancer without potentiating even more off-target toxicity to normal tissues can be improved.